FILE PHOTO: Signage from an old border customs and excise hut is seen in the border village of Glenfarne, Ireland, February 21, 2018. REUTERS/Clodagh Kilcoyne

LONDON (Reuters) - The European Union wants a customs border in the Irish Sea, between Northern Ireland and the rest of the United Kingdom, in the event of a no-deal Brexit, a letter from Theresa May seen by the London Times newspaper suggested.

According to the Times, May wrote to the leaders of the small Northern Irish political party she relies on to govern, saying that the EU is still pushing for a so-called “backstop to the backstop” which would keep the province in regulatory alignment with the Republic of Ireland to avoid a hard border.

In the letter May said she would never allow a divide between the province and Britain “to come into force”. The Times said this had been interpreted by the Democratic Unionist Party as a sign that the clause will be inserted into the legally binding agreement.

Arlene Foster, the head of the DUP, said May’s letter “raises alarm bells for those who value the integrity of our precious Union and for those who want a proper Brexit for the whole of the UK”.

May has been battling to agree a divorce deal with the EU that can gain support in Brussels, within her own deeply divided party and with the DUP who do not want Northern Ireland to be treated differently to the rest of the UK.

A spokesman for May’s Downing Street office said the letter set out the prime minister’s commitment to never accepting a situation where the UK is divided into two customs territories.

“The government will not agree anything that brings about a hard border on the island of Ireland,” he added.

(The story was refiled to fix typos in the headline and first paragraph)